2.4.1 Indicators of biomass energy content

In order to establish biomass energy systems, the energy content of each type of biomass feedstock should be determined at first. Heating value is often used as an indicator of the energy that biomass contains. Heating value is the amount of heat generated when a substance undergoes complete combustion, and is also called the heat of combustion. Heating value is determined by the ratio of components and the kinds and ratios of elements (especially carbon content) in biomass.

(a) Higher and lower heating value

Biomass comprises organic substances composed mostly of carbon, hydrogen, and oxygen, and when it is completely combusted, it produces water and carbon dioxide. The generated water and water vapor contain much latent heat that is given off upon condensation. The heating value which includes latent heat is the high heating value (HHV), while that from which latent heat is subtracted is the low heating value (LHV).

(b) Available heat

Heating value Q0 is the amount of heat that arises from complete combustion per unit material under standard conditions. Actual biomass contains much moisture and ash, which must be taken into consideration when energy is producing. Merely evaluating low heating value is inadequate as an indicator whether biomass in its natural state will sustain combustion. The energy to raise ambient air to the temperature that maintains the fire and the endothermic energy of ash should also be taken into account. The heat amount that takes them into account is the available heat, and is expressed with this equation:

Available heat Q = Q0 (1 — w) — 1000 w — [flue heat absorption] — [ash heat absorption]

(w: moisture content)

Fig. 2.1.3. uses values calculated for available heat Q at 900°C. A positive (+) value for available heat Q is the condition for combustion to occur.